Non-single crystal maser with arbitrary magnetic field direction



Jan. 30, 1968 E1 s. SABISKY ET AL 3,366,887

NON-SINGLE CRYSTAL MASER WITH ARBITRARY MAGNETIC FIELD DIRECTION Filed Sept. 4, 1962 2 Sheets-Sheet 1 IN ENTORS fif/VA) 1 51405 fommo 5. JAE/5K) Ara/M- Jan. 30, 1968 E 5 5 ET AL 3,366,887

NON-SINGLE CRYSTAL MASER WITH 'ARBITRARY MAGNETIC FIELD DIRECTION Filed Sept. 4, 1962 2 Sheets-Sheet 2 l l 1 W v W I I I Q 0 o N \o m g cs 2 i 3 Q/($99;

. INVENTORS HENRY K [en/1.5 5 g I I fomwosf/w/szr M. Ma

United States Patent C) T 3,366,887 NON-SENGLE CRYSTAL MASER WITH ARBITRARY MAGNETIC FIELD DIRECTION Edward S. Sabisky, Trenton, and Henry R. Lewis, Princeton, N.J., assignors to Radio Corporation of America, a corporation of Delaware Filed Sept. 4, 1962, Ser. No. 221,297 Claims. (Cl. 330-4) This invention relates to a maser, and particularly to maser employing a novel maser element.

A maser includes, generally, a maser element in the form of a single crystal having a minor proportion of impurity ions, which ions provide three or more energy levels of interest. A three-level maser includes a microwave pump that produces an emissive electron population distribution in the impurity ions in a crystal of a paramagnetic salt to which a constant magnetic field is applied. A requirement of a three-level maser is that transitions be allowed between nonadjacent energy levels. This is achieved in one three-level maser, as exemplified by systems using a single crystal of ruby or rutile doped with chromium as the maser element, by the mixing of the electronic spin states by a non-cubic crystal field. It has also been suggested that this may be achieved in another threelevel maser by the use of certain rare earth impurity ions in a cubic crystal field where a gamma, quartet of energy levels lies lowest.

Both of the foregoing systems require that the applied magnetic field be oriented in a particular direction with respect to the crystal axes of the maser element. Hence, the practical embodiments require the use of a single crystal body as the maser element, which is oriented in a particular way in the applied magnetic field.

An object of this invention is to provide in a maser, a maser element which does not require a particular orientation of the applied magnetic field with respect to the crystal axes of the element.

A further object is to provide in a maser, a maser element which need not be a single crystal.

A feature of the invention is a maser element comprising a mass of crystals of a cubic host material containing a minor proportion, preferably between 0.001 and 1.0 mol percent, of impurity ions. The impurity ions include a nucleus and an energy level which is a doublet. The two sublevels of the doublet are separated in energy by at least 1 kmc. by virtue of a hyperfine interaction with the nucleus; that is, by virtue of an interaction of the magnetic moments of the electrons with that of the nucleus. Since the splitting of the energy doublet results from a nuclear interaction and not from an interaction with the crystal field of the host material, each crystal operates equally well in all orientations of its crystal axes with respect to the direction of the applied magnetic field. Hence, the maser element may be a mass of randomly-oriented crystals.

A more detailed description of the invention and illustrative embodiments thereof appear below in conjunction with the drawing in which:

FIGURE 1 is a perspective view cavity type maser, and,

FIGURE 2 is an energy level diagram of a paramagnetic material of the kind suited for use in the maser shown in FIGURE 1 according to the invention.

A maser according to the invention comprises a maser element which is described in detail below, means for applying a substantially constant magnetic field to the maser element, means for pumping the maser element at microwave frequencies, and means for deriving an output of microwave energy from the element. Where the maser is used as an amplifier, the maser includes also means for applying a microwave signal to the maser element. The

of a typical resonant 3,366,887 Patented Jan. 30, 1968 maser may be of the resonant cavity type, as exemplified by the apparatus described in US. Patent 3,001,141 issued Sept. 19, 1961, to R. C. Fletcher et 3].; or may be of the traveling wave type, as exemplified by the apparatus described in US. Patent 3,004,225, issued Oct. 10, 1961, to R. W. De Grasse, et al.

FIGURE 1 illustrates a typical resonant cavity type maser 10. The maser 10 comprises a cavity 11 which is resonant at a pair of frequencies, one of which couples to the pumping frequency f and the other to the signal frequency f The cavity houses a maser element 12 whose composition is discussed in detail below. It is advantageous generally, to provide means for maintaining the maser element at a low temperature, for example, close to that ol liquid helium, or liquid neon. Typically, the cavity is enclosed within a suitable refrigerating apparatus 11A which is shown schematically in the interest of smplicity.

Pumping power of appropriate frequency f is applied to the cavity 11 from a local oscillator 13 by way of a coupling loop 14 in a manner known to workers in the art for exciting the corresponding resonant mode of the cavity 11. Input signal power of appropriate frequency f is supplied to the cavity 11 from a signal source 15 by way of a coupling loop 16. It is desirable to include a signal source isolator 17 in the signal path intermediate between the input signal source 15 and the cavity 11 to minimize the transfer of power from the cavity 11 to the source 15.

Output power is abstracted from the cavity 11 for use by a load 18, which in some instances may be another maser, by a coupling loop 19. It may be desirable to include a load isolator 20 along the signal path intermediate between the load 18 and the cavity 11 to minimize the reflection of power from the load 18 back into the cavity 11. Such load isolator 20 is also advantageously maintained at a low temperature in the interest of generating low noise.

Each of the loops 16 and 19 for the signal input and for the output is arranged to couple to the cavity 11 in a mode resonant at the signal frequency f in a manner known to workers in the maser art. Alternatively, it is feasible to use only a single coupling loop to the cavity 11 which leads to one arm of a circulator, other arms of which are connected to the signal source and to the load respectively in a manner known to workers in the maser art. Similarly, only a single signal output coupling loop is necessary if the maser is employed as an oscillator. In such operation, noise arising in the walls of the cavity 11 stimulates from the maser element 12 the emission of radiation which gives rise to oscillations.

There is applied to the maser element 12 a steady or substantially constant magnetic field H which produces Zeeman splitting of the energies of the electron spin population of the material of the maser element 12. The substantially constant magnetic field may be produced, for example, from pole pieces 21 which are shown broken away in FIGURE 1.

The maser element 12 comprises a mass of randomly oriented crystals of a cubic host material containing a minor proportion of impurity ions. The host material may be any material which crystallizes in the cubic system. Some suitable host materials are: alkali halides, such as NaCl, KBr, and LiF; alkaline earth halides such as CaF BaF BaCl SrCl and Mgl or other cubic materials such as MgO, ZnS, or MgAl O The impurities are present in the host materials preferably in proportions between 0.00l and 1.0 mol percent of the host material. The impurities may be selected from the lanthanide series, such as Ho Br, and Tm; or from the actinide series, such as U.

The impurities and the host material are related to one another in that, when the impurities are in the host material, they include a doublet in the lowest energy level by virtue of a hyperfine interaction with its nucleus, so that the sublevels of the doublet are separated in energy by at least 1 kilomegacycle (kmc.), when a zero magnetic field is applied to the maser element 12. A hyperfine interaction is an interaction of the magnetic moments of the electrons of the impurity ions with the magnetic moment of the nucleus of the ions. Most previously known paramagnetic ions have hyperfine splittings of a few hundred mc. and therefore are not of interest for use in maser elements.

The maser element 12 is polycrystalline and may be a mass of loose crystals, or may be a mass of compacted crystals, or may be a mass of crystals held together with a binder, or may be a sintered mass of the crystals. In each of these cases, the crystals are usually randomly oriented with respect to one another, but could be oriented in some manner. It is preferred that the crystals occupy as high a proportion per unit volume of the maser element 12 as possible in order to achieve the optimum efiiciency.

A polycrystalline maser element comprising a mass of randomly oriented crystals has advantages over elements comprising a single crystal in reducing the complexity of the device. Small crystals are much easier to prepare than large single crystals which are suitable for maser action. Furthermore, a larger filling factor is possible in slowing structures, such as masers of the travelling wave type, with the use of a polycrystalline maser element. Also, complicated or extended structures are possible because of the flexibility in designing the shape of a polycrystalline maser element. Also, in a traveling wave device, economical use of the magnetic field may be obtained by coiling one or more of the structures.

In one example, the maser element 12 comprises a body of compacted crystals of calcium fluoride containing 0.05 mol percent of divalent holmium H FIG- URE 2 is a plot of the energy levels of Ho in CaF as a function of magnetic field H. The normally allowed transitions are thus for AM =i1, Am =0. The normally forbidden transitions AM =:1, AM =:1 are strongly allowed up to fields of a few kilogauss. For maser applications, the center three energy levels labeled El, E2, and B3 are of interest. However, this is not the only set of levels which can be used for frequencies below 1.5 kmc.

In this example, the maser element 12 is placed in the cavity 11. A constant magnetic field H is applied to the maser element 12 from the pole pieces 21. Input microwave power of a frequency f corresponding to the energy separation of levels E1 and E3 is applied to the maser element 12 from the source of pumping power 13. The pumping produces, through various transitions, a greater population of electrons at the energy indicated by the E2 level than that indicated by the E1 level. Upon relaxation, a microwave output or emission of a frequency f corresponding to the separation of the levels E2 and E1 is produced and passed to the load 18 through the load isolator 20. The microwave output or emission from the maser element 12 may be stimulated by applying to the maser element 12 a microwave signal of frequency f corresponding to the frequency of the separation of the levels E2 and E1. The signal originates in the signal source 15 and passes through the signal isolator 17 to the cavity 11. In this example, the magnetic field H is about 2.5 kilogauss, the pumping frequency f is about 12 kmc. and the signal frequency f is about 2.5 kmc. The maser element is maintained at liquid helium temperatures. The measured line width for single crystals of calcium fluoride with nominal dopings of 0.05 and 0.1 mol percent divalent holmium is 85 me. and 115 me. respectively at 42 K. A powder made of calcium fluoride containing 0.05 mol percent divalent holmium has a slightly smaller line width than a single crystal of the transition. For the sample at 15 K., the time T is about 1,000 times longer than at 4.2 K. This implies that, at these low temperatures, the relaxation time is not due to the direct process, but to an indirect or two-phonon process in which one phonon is absorbed and another emitted at a frequency differing bythe frequency of the spin.

transition.

What is claimed is:

1. A maser including a maser element comprising a mass of crystals of a cubic host material containing a minor proportion of impurity ions, said impurity ions including a nucleus and an energy level which is a doublet, the two sublevels of said doublet being separated in energy by at least 1 kmc. by virtue of an interaction of the magnetic moments of the electrons of said impurity ions with the magnetic moment of said nucleus, means for applying a substantially constant magnetic field to said element, means for pumping said element at microwave frequencies, and means for deriving an output of microwave energy from said element.

2. A maser including a maser element comprising a mass of randomly oriented crystals of a cubic host material containing between 0.001 and 1.0 mol percent of cation impurities, said impurities having a nucleus and a plurality of energy levels, the lowest of said energy levels being a doublet whose sublevels are separated in a energy by at least 1 kmc. by virtue of a hyperfine interaction with said nucleus, means for applying a substantially constant magnetic field to said element, means for pumping said element at microwave frequencies, means for applying a microwave signal to said element, and means for deriving an output of microwave energy from said element.

3. A maser including a maser element comprising a mass of densely packed, randomly oriented crystals of a cubic host material selected from the group consisting of fluorides, chlorides, bromides and iodides of alkali metals, alkaline earth metals including barium and combinations thereof, said host material containing a minor proportion of cation impurities selected from the group consisting of rare earth elements, actinide elements, and combinations thereof, said impurities having a nucleus and a plurality of energy levels, the lowest of said energy levels being a doublet whose sublevels are separated in energy by at least 1 kmc. by virtue of a hyperfine interaction with said nucleus, means for applying a substantially constant magnetic field to said maser element, means for pumping said maser element at microwave frequencies, means for applying a microwave signal to said maser element, and means for deriving an output of microwave energy from said maser element.

4. A maser including a maser element comprising a mass of densely packed, randomly oriented crystals of cubic calcium fluoride, said calcium fluoride containing between 0.001 and 1.0 mol percent of cation impurities selected from the group consisting of rare earth elements, actinide elements, and combinations thereof, said impurities having a nucleus and a plurality of energy levels, the lowest of said energy levels being a doublet whose sublevels are separated in energy by at least 1 kmc. by virtue of a hyperfine interaction with said nucleus, means for applying a substantially constant magnetic field to said maser element, means for pumping said maser element at microwave frequencies, means for applying a microwave signal to said maser element, and means for deriving an output of microwave energy from said maser element.

5. A maser including a maser element comprising a mass of densely packed, randomly oriented crystals of cubic calcium fluoride, said calcium fluoride containing a minor proportion of divalent holmium, means for apply- 6 OTHER REFERENCES Bogle et al.: Australian Journal of Physics, March 1959, pp. 1-20.

Bostick et al.: Proceedings of the IRE, February 1962, pp. 219-220.

Braunstein: Physical Review, Aug. 15, 1957, pp. 1195-1196.

Johnson et al.: Proceedings of the IRE, January 1962, pp. 86-88.

King et al.: Journal of Applied Physics, November 1959, pp. 1844-1845.

Kiss: Proceedings of the IRE, June 1962, pp. 1531- 1533.

Porto et al.: Proceedings of the IRE, June 1962, pp. 1542-1544.

ROY LAKE, Primary Examiner.

D. R. HOSTETTER, Assistant Examiner. 

1. A MASER INCLUDING A MASER ELEMENT COMPRISING A MASS OF CRYSTALS OF A CUBIC HOST MATERIAL CONTAINING A MINOR PROPORTION OF IMPURITY IONS, SAID IMPURITY IONS INCLUDING A NUCLEUS AND AN ENERGY LEVEL WHICH IS A DOUBLET THE TWO SUBLEVELS OF SAID DOUBLET BEING SEPARATED IN ENERGY BY AT LEAST 1 KMC. BY VIRTUE OF AN INTERACTION OF THE MAGNETIC MOMENTS OF THE ELECTRONS OF SAID IMPURITY IONS WITH THE MAGETIC MOMENT OF SAID NUCLEUS, MEANS FOR APPLYING A SUBSTANTIALLY CONSTANT MAGNETIC FIELD TO SAID ELEMENT, MEANS FOR PUMPING SAID ELEMENT AT MICROWAVE FREQUENCIES, AND MEANS FOR DERIVING AN OUTPUT OF MICROWAVE ENERGY FROM SAID ELEMENT. 